The present invention relates to a novel process for reducing the inherent low shear viscosity of kaolin clays contaminated with expandable layer minerals. A water soluble tetraalkyl ammonium compound is contacted with the clay during conventional wet processing. The invention relates also to novel kaolin products prepared by selectively intercalating contaminating expandable layer mineral impurities in kaolins with a low molecular weight quaternary ammonium cation.
The art of kaolin clay processing entails the controlled mining, blending and wet processing of a variable natural resource into high quality industrial products. Procedures used by the industry in the wet processing are known in the art.
Typically, the kaolin clay crude is crushed and made down into a slurry form (blunged in water) at the mine site with the aid of one or more anionic dispersants. After this blunging step, the pH of the dispersed slurry is usually about 7 to 9. Oversized particles (grit) consisting largely of sand particles are then removed from the blunged clay by known means such as the use of screens and gravity settling. In many processing operations, the degritted slip of kaolin is then separated into one or more particle size fractions by centrifugation or sedimentation. Well known methods are then used to increase the brightness of the fractionated or unfractionated kaolin clays. The brightness improvement methods employed on a clay process stream typically include magnetic separation and reductive bleaching, and may include flotation, and ozonation/oxidative bleaching. A delamination step may be carried out before bleaching. Other than the step of reductive bleaching, which is carried out at an acidic pH, the beneficiation steps are generally conducted with the kaolin slurry in a dispersed state, with the slurry pH neutral to basic. Removal of solubilized impurities along with by-products salts is effected by dewatering the bleached clay, usually by filtration, following by rinsing with clean water.
It is well known that the low shear viscosity of kaolin clay crudes and of the particle size fractions derived from the crudes is one of the principal physical characteristics determining whether or not and, if so, in what proportions the clays and derived fractions will be utilized in the production of high value-added products for the paper industry. See, for example, U.S. Pat. No. 4,105,466 and U.S. Pat. No. 4,030,941, Kunkle, et al.
It has been reported that the low shear rheology of kaolin clay is affected by the surface area of the kaolin particles, with high surface area resulting in high values of low shear viscosity, while high shear viscosity is influenced by particle morphology and particle packing characteristics. See Murray, Haydn, (1984), "Clay" in Pigments for Paper, Robert Hagemeyer, editor, TAPPI Press, 95-143. It has long been known that expanding minerals, such as montmorillonite, hectoritc, degraded illite and illite/smectite have an adverse effect on the inherent low shear viscosity of kaolin clays. Sodium montmorillonites are particularly troublesome. Generally, these expandable minerals are slimed and can be concentrated in an ultrafine fraction of a kaolin crude (e.g., a fraction 0.2 microns or finer). Accordingly, several suggested techniques for reducing the low shear viscosity of kaolin clay involve physically removing mineral slimes from the kaolin particles. For example, application of high shear followed by centrifugation has been proposed in U.S. Pat. No. 3,464,634, Brociner. Halaka, et al., U.S. Pat. No. 5,128,027, teach a method to remove mineral slimes by overdispersing a kaolin slurry, thereby reflocculating kaolin particles, and then allowing the kaolin flocculates to settle and separate.
U.S. Pat. No. 4,105,466 and U.S. Pat. No. 4,030,941, Kunkle, et al., relate to processes for reducing the clay-water viscosity of crude kaolins having "inherent" high low shear viscosities. The processes feature the incorporation of an aluminum hydroxide polymer with the clay during wet processing and prior to the filtration step. Washing and drying are optional. U.S. Pat. No. 4,045,235, Bidwell, et al. describes similar processing of kaolin with the same type of inorganic polymer for the same purpose. The aluminum hydroxide polymer is identified as being positively charged. Bidwell, et al. note that reaction is "apparently not with the clay mineral itself, e.g., kaolinitc . . . but the impurities which are associated with the clay mineral, e.g. montmorillonite".
Numerous patents disclose the addition of reagents to finished clay slurries to reduce viscosity and/or to stabilize viscosity of finished slurries. Patentees do not take steps to improve the inherent viscosity and do not introduce chemicals to control viscosity during wet processing. For example, in Malden, U.S. Pat. No. 4,106,949, a polymeric organic base (polyamine) is added to a dilute clay water slurry to reduce viscosity. Rowland, U.S. Pat. No. 2,994,616 incorporates a water soluble guanidine salt.
It is well known that when cationic materials such as quaternary ammonium salts are added to anionically dispersed kaolin slurries flocculation and thickening occur as a result of charge neutralization. This phenomenon is utilized in the preparation of so-called "bulking" pigments by employing salts of cationic polymers that have a high charge density. See, for example, U.S. Pat. No. 4,738,726, Pratt, et al. and Nemeh, et al., U.S. Pat. No. 4,772,332, both assigned to the assignee of the subject patent application. The cationic polymer is added during wet processing, for example, prior to or after bleaching, and is retained in the filtered washed product. The bulked product has significantly higher viscosity than the original kaolin. It is also known that the same cationic polymers used by Pratt, et al. to flocculate kaolin can deflocculate and thus fluidize kaolin slurries if the cationic is utilized in appropriate concentration and at an acidic pH in the absence of anionic dispersants. See U.S. Pat. No. 5,061,461, Sennett, et al. which utilizes a high charge density cationic polymer in this manner during wet processing of kaolin crudes.
U.S. Pat. No. 3,235,666, Sawyer, et al., assigned to a predecessor of the assignee of the subject patent application, discloses that quaternary ammonium salts of the type having germicidal properties increase the viscosity of kaolin slurries. The salts are not added during wet processing.
It is known that low molecular weight quaternary ammonium compounds form intercalation complexes with expanding lattice minerals but not with kaolin minerals. Theng, et al., Clay Minerals (1967) 7, 1-17; ibid, (1968) 271-293. See also A. Weiss, Angew. Chem. internat. Edit. 2, 134-144 (1963).